The headword is an adjective in the masculine nominative singular; see LSJ s.v. It is first attested at Pindar, Pythians 2.96 (web address 1), where it is used as a metaphor for hazardous or treacherous.
[1] The first gloss, which is (or at least can be) synonymous with the headword, is a two-ending adjective in the masculine and feminine nominative singular; see LSJ s.v. The second gloss highlights a metaphorical usage of the headword, to mean elusive, evasive, or untrustworthy. An instance of this sense (II.1 in LSJ s.v.), involving the headword's superlative form o)lisqhro/taton (neuter nominative singular), occurs at Plato, Sophist 231A (web address 2), where the Eleatic Visitor reminds Theaetetus that resemblances are a most slippery kind. The headword is identically glossed in the Synagoge, Photius' Lexicon (omicron219 Theodoridis), Lexica Segueriana 316.14.